1. Field of the Invention
The present invention relates to image processing apparatuses and image processing methods that carry out image processing on an image.
2. Description of the Related Art
Image reading apparatuses that obtain image data by capturing an image of a document on a document platform from directly above or at an angle from above using an image capturing unit such as a camera, a scanner, or the like have been known for some time, as represented by document cameras. However, distortion occurs in the read image when the distance from the image capturing unit is not constant, and thus it is necessary to correct the distortion. To obtain post-distortion correction process pixel values for given pixels in such a distortion correction process, it is necessary to obtain relative post-distortion correction process coordinates of the pixel in the pre-correction process image, as well as carrying out an interpolation process that finds grid point pixel values from non-integer coordinate pixels (that is, non-grid point pixels) that have coordinate components below a decimal point and are thus not integer coordinates (coordinates whose coordinate components are expressed as integers; also called grid points) found as a result of obtaining the relative coordinates. The nearest neighbor method, which substitutes the pixel values from the nearest pixels, is an example of such an interpolation method. However, this method is problematic in that jaggies, line dropouts, and so on will arise. Accordingly, a technique is known in which a convolution operation is carried out by using a sampling function and referring to the pixel values of a plurality of pixels within a limited region in the periphery of the given pixel, and estimating the pixel value at those coordinates (see, for example, Japanese Patent Laid-Open No. 11-353473).
With this interpolation process that uses a convolution operation, the image quality that is achieved will differ depending on the breadth of the region subjected to convolution, the frequency characteristics of the sampling function that is used, and so on. In other words, generally speaking, a broader reference region relative to the pre-processing image or a lower frequency for the sampling function results in a greater smoothing effect being produced on the original image and less jaggies in edge areas. Conversely, a narrower reference region relative to the pre-processing image or a higher frequency of the sampling function results in a greater sharpening effect being produced on the pre-processing image and more jaggies in edge areas.
Meanwhile, when a non-planar document is placed on a document platform and captured by a camera located directly thereabove, the tilt of the document relative to the document platform, or in other words, the angle relative to the optical axis of the camera's optical system, will differ from location to location in the document, and thus there is a property that the frequency characteristics of content printed on the document differ from location to location. In other words, in the case where a booklet having a degree of thickness, such as a magazine or a dictionary, is placed in an open state on the document platform with its pages facing upward, a region near the binding between the pages will appear to have a higher frequency component from the viewpoint of the camera as compared to a region near the center of each page, even if the frequency components are the same when the pages are printed. Meanwhile, even if the tilt of the document relative to the document platform is constant, the captured image may distort in, for example, a barrel shape due to optical distortion present in the camera's optical system itself. In this case, even if the frequency characteristics are uniform throughout the plane when the document is printed, the frequency characteristics of the captured image will differ from location to location.
If the aforementioned interpolation process that employs a convolution operation using a sampling function is applied uniformly to an obtained image having such characteristics, the relative breadth of the reference region and the frequency characteristics will change depending on the location within the image. As a result, unevenness appears in the effect on the post-distortion correction process image. In other words, when carrying out an interpolation process on a captured image of a thick booklet document as mentioned earlier, even if a distortion correction process is applied uniformly, the sampling function will have a relatively high frequency in regions near the center of each page due to the image obtained at those regions having a low frequency, resulting in a sharp state, whereas the sampling function will have a relatively low frequency in regions near the binding due to the image obtained at those regions having a high frequency, resulting in a smoothed state.